For the production of hydrocarbon wells, boreholes are drilled into subterranean formations. Following standard procedures, a fluid is circulated during drilling from the surface through the interior of the drill string and the annulus between drill string and formation. The drilling fluid, also referred to as “drilling mud”, is used to accomplish a number of interrelated functions. These functions are:    (1) The fluid must suspend and transport solid particles to the surface for screening out and disposal;    (2) It must transport a clay or other substance capable of adhering to and coating the uncased borehole surface, both (a) to exclude unwanted fluids which may be encountered, such as brines, thereby preventing them from mixing with and degrading the rheological profile of the drilling mud, as well as (b) to prevent the loss of downhole pressure from fluid loss should the borehole traverse an interval of porous formation material;    (3) It must keep suspended an additive weighting agent (to increase specific gravity of the mud), typically barites (a barium sulfate ore, ground to a fine particular size), so that the entire column of drilling fluid is not interrupted upon encountering pressurized pockets of combustible gas, which otherwise would tend to reduce downhole pressure, as well as creating a “blowout” in which the fluid and even the drill stem are violently ejected from the well, with resulting catastrophic damages, particularly from fires;    (4) It must constantly lubricate the drill bit so as to promote drilling efficiency and retard bit wear.
The industry distinguishes between largely three classes of drilling fluids: oil-based, water-based and so-called synthetic muds. Oil-based and synthetics muds are recognized for their superior qualities for most of the drilling operations.
A drilling fluid typically contains a number of additives. Those additives impart desired properties to the fluid, such as viscosity or density. One class of additives is used as fluid loss agents to prevent the drilling fluid from entering into porous formations. While fluid loss agents are designed to form a competent filter cake to cover porous formations, there is another class of materials referred to as “lost circulation” agent. Lost circulation materials are designed to prevent or limit fluid losses into fractures or other openings in the formation.
The loss of drilling fluid into fractures is a major concern in the well construction process. There are a large number of products either employed, tested or proposed for reducing these losses. These products are typically particulates that invade the fracture and at some point along the fracture form a blocking bridge across the fracture. When the fracture is bridged a filter cake will develop due to the pressure difference between the wellbore fluid and the fluid beyond the fracture block. The pressure beyond the filtercake or bridge block is reduced and thus may prevent the full borehole pressure from affecting the tip of the fracture. However any further increase in the wellbore fluid pressure may result in further opening of the fracture leading to renewed losses. The additive described here is effective at blocking a fracture and provides resiliency that helps prevent re-opening of the fracture.
The US published patent application 2004/0033905 provides a drilling fluid composition comprising a drilling fluid vehicle, which may be oil- or water-based, and a composite microsphere component. The microspheres of the composite microsphere component may comprise any hollow microspheres of glass, ceramic or plastic that may be added to the drilling fluid (with other components of the drilling fluid known in the art) to reduce the density thereof.
The U.S. Pat. No. 4,579,668 describes a lost circulation agent which comprises a blend of two particulate materials. One or both of the materials may be derived by the processing of discarded wet-cell battery casings, which are stated to be principally made of polypropylene. The first particulate material employed is a thermoplastic polymer in a flexible, elongated laminar form having the desirable properties of both fibers and flakes. And the second of the particulate materials is a granular, thermoset plastic which has been ground into a granular form. This granular material may be derived from the processing of wet-cell batteries, as noted previously, in which the cases are formed of a hard rubber such as the phenol-formaldehyde resin available under the trade name Bakelite. As will be understood by those skilled in the art, that hard, synthetic rubbers such as Bakelite are thermoset resins which have a cross-linked polymeric network and remain relatively hard under high temperature conditions. The thermoset polymers may be contrasted with the thermoplastic polymers which are linear or branched chain polymers and which are softened by the application of heat. The thermoset resin preferably is ground in a distribution across a relatively wide particle size range to provide a maximum density bed particle size distribution as described in greater detail hereinafter. The particulate granular material has a specific gravity within the range of 1.2-1.4.
The use of graphitic materials for fluid loss and lubrication purposes is disclosed in the U.S. Pat. No. 5,826,669. A resilient graphitic carbon particle drilling fluid additive is used that is effective at preventing and controlling loss of drilling fluid into subterranean formations and has good lubrication properties. It showed that pores and fractures in shales, sandstones, and the like are effectively sealed with resilient graphitic carbon particles that can be tightly packed under compression in the pores and fractures to expand or contract without being dislodged or collapsing due to changes in the equivalent circulating density or with an increase in fluid weight. The patent does not recognize the importance of the re-opening pressure as a parameter to select and optimize lost circulation agents.
It is therefore an object of the present invention to provide novel agents for drilling fluids, particularly suitable for use as lost circulation agent.